CLEAN WITH VINEGAR AND BAKING SODA
Get an old cleaning toothbrush and dip the head of it in vinegar then baking soda. Or just mix the vinegar and baking soda together to create a paste-like mixture. Use this mixture to scrub in between the tiles. Then rinse off the mixture with warm water.
SCRUB WITH DISH SOAP
Squeeze a few drops of dish soap on your toothbrush and use it to scrub the tile grout. Be sure to scrub everywhere.
SCRUB WITH SCRUBBING BUBBLES
For this, all you have to do is spray a can of scrubbing bubbles in between the tiles. Then let the scrubbing bubbles residue sit there for 10-30 minutes; after that just get an old toothbrush and scrub.
USE HYDROGEN PEROXIDE AND VINEGAR MIXTURE
Get a clean damp cloth and pour 2 cups of hydrogen peroxide on it. Then use the cloth to clean in between the tiles.